1. Field of the Invention
The present invention relates to a pulverized catalyst for use in the polymerization of alpha-olefins such as ethylene to obtain polymers of improved flow characteristics and higher bulk density and to the process of polymerizing alpha-olefins using the catalyst.
2. Related Art
It is well known that the alpha-olefins such as ethylene can be polymerized by contacting them under polymerization conditions with "Ziegler" type catalyst, i.e., a catalyst obtained by activating a Group 4b or 5b transition metal-containing catalyst component with a cocatalyst, e.g., a titanium compound activated with an organometallic compound. It is further known that the use of such catalyst particles provide "templates" for the formation of the polymer particles, the size and size distribution of such polymer particles being largely dependent upon the size and particle size distribution (PSD) of the catalyst being used. It is further known that the characteristic of the polymer, i.e., irregular morphology and wide particle size distribution are replicated in the polymer produced with the catalyst.
U.S. Pat. No 4,143,223 discloses the preparations of Ti containing catalyst by mechanically pulverizing a magnesium compound and organic ester and impregnating the resultant solid with TiCl.sub.4 which was then used with an alkyl aluminum cocatalyst to polymerize an alpha-olefin.
With certain olefin polymerization catalysts, the growth of the catalyst itself from the original catalyst seeds can be controlled to yield products having a coarse structure (20 microns or larger) which makes these catalysts more easily handled. Thus, A. P. Haag and M. Weiner (U.S. Pat. No. 3,623,846, issued Nov. 30, 1971, assigned to Dart Industries, Inc.) described a process for controlling particle size during condensation and/or desublimation of a material such as titanium trichloride which may be used in the polymerization of alpha-olefins.
In another example as described in British Pat. No. 1,139,450, assigned to Shell Internationale Research Maatschappij, TiCl.sub.3 catalysts are formed by controlled reduction of titanium tetrachloride with aluminum alkyls. These materials have a narrow PSD, and have an average diameter greater than 15 microns, and therefore are relatively easy to handle.
It is known that solid magnesium compounds mechanically pulverized with an organic ester and impregnated with a titanium compound produce a titanium containing catalyst component for use with organometallic compounds in Ziegler type polymerization of alpha-olefins and that the resultant polymers are highly stereoregular. However, pulverized catalysts are known to have irregular morphology and wide particle size distribution. Thus, polymer produced from these catalysts have poor flow properties and low bulk density as a result of the irregular particle size of the polymer.
U.S. Pat. No. 4,224,185 discloses solid catalyst particles of titanium halide are made into a form and shape that is easier to handle, without changing their surface area or adversely affecting their activity by "matting" the catalyst particles with a fibrillatable polymer by mechanical shearing action and subsequently shaping the "mat", followed by controlled sizing and shaping into the form of pellets, tablets, and the like by known mechanical means. U.S. Pat. No. 4,220,555 discloses a similar composition with the difference that a non fibrillatable polyolefin is also employed.
British Patent Specification No. 871,828 discloses the preparation of alpha-olefin polymerization catalyst by polymerizing an alpha-olefin with a titanium trichloride and comminuting the resultant reaction mixture to produce catalytically active particles.
According to the present invention it has been found that particulate Group 4b or 5b transition metal containing catalyst (preferably pulverized), impregnated in solid polyethylene and subjected to conditions of shear at the softening point of the polyethylene in an appropriate solvent, when used as a Ziegler type catalyst component, produces polymer powder of better flow and higher bulk density than conventional mechanically pulverized catalyst components.
An advantage of the present catalyst component is that the polymer powder replicated therefrom is of more regular morphology and even particle size distribution with less fines. A particular advantage is the handling of the bulk polymer powder is easier since it flows more easily and does not tend to clog or block equipment. A further advantage is that the regular particle size of the polymer powder provides a larger volume of particles in the same space than irregular sized particles (i.e., higher bulk density).